It is looking at User-Agent header for "Windows NT\ ?\d\.\d" string and only listed known recent Windows NT versions (from Windows XP), otherwise it will return

It is looking at User-Agent header for "Windows NT\ ?\d\.\d" string and only listed known recent Windows NT versions (from Windows XP), otherwise it will return "Windows 8.1". Tested this with: curl -H "User-Agent: Windows NT 5.1" http://windows.microsoft.com/en-us/windows/which-operating-system Results: NT 5.1 - Windows 8.1 WindowsNT 5.1 - Windows 8.1 Windows NT 5.1 - Windows 8.1 Windows NT - Windows 8.1 Windows NT5.1 - Windows XP Windows NT 5.0 - Windows 8.1 Windows NT 5.1 - Windows XP Windows NT 5.2 - Windows 8.1 Windows NT 6.0 - Windows Vista Windows NT 6.1 - Windows 7 Windows NT 6.2 - Windows 8 Windows NT 6.3 - Windows 8.1 Windows NT 6.4 - Windows 8.1 Windows Linux NT 5.1 - Windows 8.1